In fiber/coaxial subscriber line networks, the availability of a stable clock signal at the reception side is indispensable for the faultless functioning of the reception-side decoder, particularly given digital, data-compressed video signals transmitted in the form of ATM signals: In an ATM system, the signal transmission fundamentally proceeds asynchronously, i.e. the individual ATM cells in fact arrive in the receiver in the correct sequence, but at irregular time intervals. Although the recovery of an adequately stable reception clock is thereby fundamentally possible, for instance by employing a large buffer memory for smoothing the data stream (with the consequence of undesired signal delay), by the co-transmission of time marks that are regularly mixed in and the like, such techniques are complicated and expensive. For the acceptance of a system, meanwhile, a cost-optimized solution specifically at the subscriber equipment is of great significance.
For the reception-side clock supply for digital signals, particularly digital, data-compressed TV distribution signals transmitted in the form of ATM signals to the subscriber-side network termination units from a digital signal source, particularly a TV signal source, via a connection unit and a light waveguide and/or coaxial line tree network connected thereto and shared by a respective plurality of subscriber-side network termination units, one can proceed such that the signal clock(s) required is (are), at any rate, separately transmitted from the connection unit as a head location to all network termination units connected to the light waveguide and/or coaxial conductor tree network. Thus, the digital signal acquired from the received ATM cells is time-regenerated (EP Application 94104201.2).
This method yields the advantage that the signal clocks can also be transmitted from the (video) signal source to the head station (associated unit) in a way (for example, with mixed-in time marks in the form of specific ATM cells) that can be connected with a higher expense for the transmission or, respectively, recovery of a stable clock signal, This can occur in common for all subscribers connected to the corresponding connection unit. By contrast, the expense exerted in subscriber outlay to be exerted subscriber individual fashion for the clock recovery can be kept relatively low.
The higher expense is thereby more likely to be bearable when signal clocks of only one or a few (video) signal sources situated, if possible, in the proximity of the head station (connection unit) are to be transmitted and recovered. When, by contrast, many (video) signal sources are to be simultaneously incorporated and/or greater distances and, potentially, telecom administration boundaries lie between (video) signal sources and head station (connection unit), then the procedure could encounter economic or technical limits as well. Digital, data-compressed video signals, however, should also be capable of being delivered via the public network from remote video servers or sources, even potentially crossing boundaries, without these having to be synchronized with one another in terms of frequency or, with a locally existing clock.
In this context, it is already known (from A. D. Gelman, S. Halfin, W. Willinger, "On Buffer Requirements For Store-And-Forward Video on Demand", Proc. GLOBECOM '91, pp. 976-980) to provide a burst-like transmission with high transmission rate (up to 150 Mbit/s) for the transmission with ATM from a remote video databank to a buffer memory in the local head station via the long-distance network, to intermediately store the information in the head station and then to slowly transmit the information to the subscriber in real time. This requires a correspondingly high memory capacity in the head station.
A version of this procedure (recited by D. Deloddere, W. Verbiest, H. Verhille, "Interaktive [sic] Video On Demand" in IEEE Communications Magazine, May 1994, pages 82-88) provides a fast, high-rate transmission and storing of only the first minutes of the film, whereas the rest is slowly transmitted and written into the buffer memory. The above-addressed problem of a low-outlay or, respectively, cost-beneficial reception clock supply even given a plurality of (video) signal sources or, respectively, given greater distances and, potentially, telecom administration boundaries lying between (video) signal sources and head station (connection unit) remains unresolved in both instances.
By comparison thereto, the invention discloses a way for an advantageous reception-side clock supply of subscriber-side network termination units of a light waveguide and/or coaxial line tree network for fiber/coaxial subscriber line networks, said network termination units calling digitally data-compressed video signals transmitted with ATM.
The invention is directed to a method for the transmission of ATM digital signal of a program unit, particularly of digital, data-compressed video distribution signals, from a remote digital signal source, particularly a video server, via a head station of a light waveguide and/or coaxial line tree network respectively shared by a plurality of subscriber-side network termination units to at least one of the subscriber-side network termination units, whereby an intermediate storage of such ATM digital signals transmitted with elevated data rate occurs in the head station; this method is inventively characterized in that the program unit is transmitted from the digital signal source to the intermediate memory of the head station with a data rate that is slightly elevated compared to the data rate nominally required for the program unit and that is retained for the entire duration of the transmission, the data stream being further-transmitted therefrom to the respective subscriber with the signal clock nominally required for the program unit that is offered by a reference clock source; the reference clock(s) is (are) thereby separately transmitted at any rate from the head station to all network termination units connected to the light waveguide and/or coaxial line tree network, whereat the digital signal acquired from the received ATM cells is thus time-regenerated.
By offering corresponding reference clock sources in the head station of a light waveguide and/or coaxial line tree network shared by a plurality of subscriber-side network termination units, the invention, which proceeds therefrom that only a limited plurality of different nominal data rates will be provided in the network, yields the advantage that, given a transmission of digital, data-compressed video signals in ATM format from remote digital signal sources (video servers) via the (potentially public) transmission network to the head station of the light waveguide and/or coaxial line tree network and, further, to the light waveguide and/or coaxial line tree network shared by the subscriber-side network termination units to the subscriber-side network termination units thereof, no signal clocks need be transmitted between digital signal source and head station and no synchronization is required between digital signal source and light waveguide and/or coaxial line tree network. Slight deviations of the video (film) speed compared to the speed in the original recording or, respectively, storing and encoding are imperceptible.